[docs] move own-instance.rst out of docs/user

docs/own-instance.rst

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+===========================
+Why use a private instance?
+===========================
+
+  *"Is it worth to run my own instance?"*
+
+\.\. is a common question among SearXNG users.  Before answering this question,
+see what options a SearXNG user has.
+
+Public instances are open to everyone who has access to its URL.  Usually, these
+are operated by unknown parties (from the users' point of view).  Private
+instances can be used by a select group of people.  It is for example a SearXNG of
+group of friends or a company which can be accessed through VPN.  Also it can be
+single user one which runs on the user's laptop.
+
+To gain more insight on how these instances work let's dive into how SearXNG
+protects its users.
+
+How does SearXNG protect privacy?
+=================================
+
+SearXNG protects the privacy of its users in multiple ways regardless of the type
+of the instance (private, public).  Removal of private data from search requests
+comes in three forms:
+
+ 1. removal of private data from requests going to search services
+ 2. not forwarding anything from a third party services through search services
+    (e.g. advertisement)
+ 3. removal of private data from requests going to the result pages
+
+Removing private data means not sending cookies to external search engines and
+generating a random browser profile for every request.  Thus, it does not matter
+if a public or private instance handles the request, because it is anonymized in
+both cases.  IP addresses will be the IP of the instance.  But SearXNG can be
+configured to use proxy or Tor.  `Result proxy
+<https://github.com/asciimoo/morty>`__ is supported, too.
+
+SearXNG does not serve ads or tracking content unlike most search services.  So
+private data is not forwarded to third parties who might monetize it.  Besides
+protecting users from search services, both referring page and search query are
+hidden from visited result pages.
+
+
+What are the consequences of using public instances?
+----------------------------------------------------
+
+If someone uses a public instance, they have to trust the administrator of that
+instance.  This means that the user of the public instance does not know whether
+their requests are logged, aggregated and sent or sold to a third party.
+
+Also, public instances without proper protection are more vulnerable to abusing
+the search service, In this case the external service in exchange returns
+CAPTCHAs or bans the IP of the instance.  Thus, search requests return less
+results.
+
+I see. What about private instances?
+------------------------------------
+
+If users run their :ref:`own instances <installation>`, everything is in their
+control: the source code, logging settings and private data.  Unknown instance
+administrators do not have to be trusted.
+
+Furthermore, as the default settings of their instance is editable, there is no
+need to use cookies to tailor SearXNG to their needs.  So preferences will not be
+reset to defaults when clearing browser cookies.  As settings are stored on
+their computer, it will not be accessible to others as long as their computer is
+not compromised.
+
+Conclusion
+==========
+
+Always use an instance which is operated by people you trust.  The privacy
+features of SearXNG are available to users no matter what kind of instance they
+use.
+
+If someone is on the go or just wants to try SearXNG for the first time public
+instances are the best choices.  Additionally, public instance are making a
+world a better place, because those who cannot or do not want to run an
+instance, have access to a privacy respecting search service.